Inside the T/S1000, ZX81

If you get the chance, look inside an IBM PC. You’ll see acres of board covered with countless chips, resistors, capacitors and the like. It all looks very mysterious and impressive. Instead of staring open-mouthed in awe and dreaming wistfully of the day you own one, comfort yourself with the knowledge that your seemingly lowly T/S1000,ZX81 is a superior technological achievement.

Basically, all computers work the same way, so studying the inner components of your T/S1000,ZX81 is an easy way to learn about computer hardware in general.

The photographs in this article may not completely satisfy your curiosity, so if you must open your computer, don’t use a crowbar; instead, remove the screws under the leg pads with a small Philips screwdriver. Now the mother board is exposed. (I haven’t a clue as to why they call it a mother board, and I’m still waiting for father boards and little baby boards to come along.)

You will probably be unimpressed by what you see. Indeed, you may wonder how anything so small and ordinary looking can function as a computer. What makes Clive Sinclair a genius is that he has managed to squeeze all that computer power into a few miniscule chips that cost very little to manufacture and are easy to test. We as consumers are the winners, getting a unique computer with features otherwise attainable only in higher-priced units.

All computers use the bit as their basic functional unit. A bit is essentially an on/off switch that is permanently set in the ROM (read-only memory) but is switchable in the RAM (random access memory). The ROM switch is maintained in position even when the power is cut-off. The ROM has the memory that tells the computer how to be a computer. In the RAM, when the power is shut off, the switches do not maintain position, hence its memory is lost.

The T/S1000,ZX81 has a ROM chip and a static RAM chip, and various parts on the board that are supportive of these chips. Alone, the RAM and the ROM are just a bunch of bits without purpose; they need an interpreter. A microprocessor is basically a program called a “microprogram”, whose function is to organize those memory bits into blocks. These blocks are then interpreted as binary numbers at definable addresses.

The heart of the T/S1000,ZX81 is the Z80 A central processing unit (CPU), the recognized leader among 8-bit microprocessors. Subsequent editions of this chip, such as the Z80 B and C, put 16-bit processors to shame. In choosing this powerful chip, Clive Sinclair has utilized chip technology to the fullest.

The Z80 A is an 8-bit processor because it refers simultaneously to 8 bits in an address. Internally, the Z80 A can deal with up to 16 bits simultaneously on a type of scratch-pad called the register pairs. With 16 bits, you can make a binary nr aber of up to 65553, and this is the maximum number of addresses the Z80 A can form. Sinclair rendered one leg of the chip disfunctional for this purpose, so the actual addressing range is 32768. The processor not only interprets the numbers it finds as data, but also the numbers it finds as instructions. This allows you to copy the contents of certain addresses onto the computer’s scratch-pad, test and process these numbers, make logical decisions affecting the computer’s next act, and write into addresses.

By organizing 8-bit numbers into a certain order in ROM or RAM, the programmer creates the “macroprogram” in machine language. The first such program to be installed resides at the beginning of memory and is called the monitor. Many computers have a small permanent program, sufficient to control screens and load full systems into RAMS. The T/S1000.ZX81 has a full monitor system located in a ROM chip.

But the master touch is the chip which holds the Sinclair logic. It acts as an intermediary between the various peripherals and chips, and also selects the memory to be accessed, ROM or RAM. In fact, the other chips need its permission to function. With this switching terminal, Sinclair was able to make a four chip machine* In the hardware illustration accompanying this article, you can see that all roads lead to the Sinclair logic.

When you power up through the 9-volt entry (located on the side), the current passes through a regulator and is stabilized at 5 volts. Heat from the process is diffused by an aluminum radiator, and escapes through slots in the bottom of the casing. Once on, the microprocessor reads the contents of the first address, then goes through an in-program initialization procedure according to the ROM program. Memory is cleared, parameters set up, and the display file is prepared — a step that starts with a ripple moving down the screen and ends when the cursor finally appears.

On the circuit board is a connector with a folded plastic strip leading under the keyboard. When you press a key, you cause two metallic strips crisscrossed beneath it to contact with each other, so creating a circuit. By reading these circuits, the processor is able to produce two 8-bit numbers, thereby distinguishing the key pressed as well as the mode the computer was in when the key was pressed. When the computer is in SLOW mode, it reads the keyboard 50 times a second, and maintains the screen image at the same rate. It executes any program in RAM when the screen is blank. In FAST mode, the Z80A is not preoccupied with these tasks, and so has more time to execute your program.

Even the most modest request programmed into the 1 or 2K of RAM located on the board generates furious activity. Not only does the Z80A control the television, read the keyboard, and interpret the commands, it constantly allocates blocks of memory to take advantage of all available space. Finally, it executes your program, but that turns out to be the least of its worries. With no supplementary RAM attached to the edge connector at the back, the operating system doesn’t reserve space for screen prints. It creates space and rearranges in-use memory into suitable locations on the fly. In this way, you get the most from the limited memory of the RAM on board.

Supplementary memory is storage space that frees the program area for Basic and machine language programs. If you consult the memory chart on this page, you will notice a blank space between the addresses of 8000 and 16000. RAM, ROM, or EPROM may be situated here, but only machine language can be executed. This space is most suited to expanding the operating system or for cartridge plug-in programs. Maybe Sinclair will decide to do something with this area later. More programs of the same calibre as the present ROM would be most welcome.

There are many peripheral devices available that plug into the edge connector and improve performance. The most notable are printers and push-button keyboards. There is also hardware on the market that works with the ear and mic plugs at the side of the T/S1000,ZX81; they improve the LOAD and SAVE functions involving your cassette recorder. Enough add-ons can make the T/S1000,ZX81 unrecognizable, but that small circuit board with its few unimpressive looking chips is where the action really is.

BASIC PROGRAMS use machine language as an interpreter. In turn, another, more primary language in the microprocessor interprets machine codes.

Where does it all stop? Which language did they use to write the microprogram in the CPU? The program must have been written on a computer without a microprocessor, so the first program of all was written directly in binary — one bit at a time. They really had no choice! If your basic program asks for any input at alt then you too have created a language. Chips are just tools that allow various interdependent languages to work together. They are convenient but not absolutely essential to computing,

Which chips to use in a computer is a practical question involving financial, technical, and marketing considerations. There is no all-purpose computer. Features you do not find useful in a given system (especially in a small computer system) are just so much wasted power for you. Sinclair could have included any number of specialized chips to increase the features available on his creation. So the T/S1000,ZX81 doesn’t come with 200 colors and an assortment of whistles and bells. Neither does the IBM. Though the T/S1000,ZX81 is fun to play with and priced like a toy, it is most definitely no toy. First and foremost, it is a learning tool — with a potential for games and business applications. You sense its potential when you look inside.